rename secop.stringio to secop.io, which includes now
also BytesIO and the common base class IOBase
+ a small fix in error handling
Change-Id: I8e305e2c164f4ed131f4b36ef45edd8bd222336d
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/26393
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
